Conventional construction machines with articulated steering, such as articulated dumpers or suspension-joint farm tractors, construction or special machines, are equipped with powershift transmissions comprising torque converters or fluid clutches. In articulated dumpers and farm tractors, an intermediate axle differential is usually inserted in the transmission output which, unblocked, allows a torque distribution between front and rear axles and blocked implements a direct through drive. In addition, a transverse differential is mounted in each axle of the vehicle.
Differentials of four-wheel vehicles are activated, via blocking clutches (differential blocks). The blocking clutches are automatically or manually engaged to block the differential according to driving parameters of the vehicle or disengaged to open or switch off the differential block.
The engagement of power shiftable differential blocks, which can be designed, for example, as hydraulically actuated multi-disc clutches, can be automated by slip controls, that is, by comparing the rotational speeds of the wheels.
DE 197 49 919 A1 has disclosed such a method. Here within the scope of a slip control, the rotational speeds of both output shafts of the differential are compared with each other. The comparison slip signals which, as soon as they exceed a threshold value, activate the engagement of the blocking clutch of the respective differential taking into account the steering angle. Another method within the framework of which the rotational speeds of the wheels are determined is the object of DE 197 48 086 A1.
The measuring of the rotational speed on the wheels in working machines and service vehicles is expensive due to the low rotational speeds that appear, and is potentially unreliable on account of the exposed position and of the hard conditions of use.
For switching off the differential block, that is, for disengaging the blocking clutch, it is known from EP 0 424 932 B1 to utilize the ratio of the torques appearing on two transmission shafts as a characteristic value and to compare the currently determined actual characteristic value with a torque ratio determined by the transmission ratio as nominal characteristic value. Based on the comparison, another setting device is used for changing the torque load of the input and/or output train, according to the deviation. With the setting device are met the shifting conditions required for loosening the blocking clutch so that after reaching the shifting condition, the blocking clutch is disengaged and the differential block is thus opened or switched off.
From EP 0 410 441 A2 is known a differential transmission for motor vehicles where a differential block can be engaged and disengaged according to driving parameters. For shifting the differential block, operation data of the motor vehicle, especially rotational speeds of at least two drive shafts, are supplied to a control unit. As mentioned above, to make the disengagement of the differential blocks possible without repeatedly having to loosen the relevant blocking clutch for measuring rotational speed differences, it is proposed to place sensors for permanent torque detection on the shafts. In a control unit state, parameters are fed, especially one coordinated with the position of the gas lever and one torque value coordinated with the selected transmission gear of the vehicle; the same as also the ratio of two torques for disengaging the differential block. By comparing the supplied values with the actual values, it is possible better to adapt the engagement and disengagement of the differential block to the current driving conditions of the vehicle.
According to the prior art, the implementation of the methods for automatic actuation of differential blocks, on account of the required sensor system, has provided disadvantageous also as technically expensive and costly, especially for use in working machines and service vehicles. Furthermore, the use of self-blocking differentials acts negatively upon the consumption of fuel.
This invention is based on the problem of outlining a method for automatic actuation of longitudinal blocks in four-wheel vehicles, particularly working machines and service vehicles, which method needs for applying its already existing parameters and does not unfavorably affect the driving state of the vehicle precisely in critical situations. In addition, the inventive method needs no added sensor system.